With a communication means such as the Internet and digital broadcasting, there has been proposed a service which delivers contents such as movie and music. Data handled in such system is digitalized, and thus copying the data is performed easily. Therefore, it is important to protect the copyright of the contents. As one of the measures to protect the copyright, there is proposed a method in which the content is encrypted prior to being delivered.
In this method, the content is encrypted, for example, by use of a common key encryption system, and simultaneously a decryption key is created to decrypt the content having been encrypted. Since it is not possible to play back the encrypted content without the decryption key corresponding thereto, illegal usage of the content can be prevented and also the content copyright holder can be protected, by managing information in a form of combination between the decryption key and conditions to use the decryption key.
In the digital broadcasting, there is copy control information as a usage condition, and usage conditions such as “Copiable”, “Copiable-for-one-generation”, and “Copy-inhibited” are put into practice. Furthermore, the contents defined as “Copiable-for-one-generation” or “Copy-inhibited”, as to which copying is limited, are protection targets. Therefore, when those contents are accumulated, it is necessary to encrypt the contents with a local encryption so as to protect the contents. Accumulating contents of “Copy-inhibited” has to be performed within a limited time of temporary storage allowable time. If the content of “Copiable-for-one-generation” is accumulated, it has to be accumulated with a condition of “Recopy-inhibited”, and copying from the content defined as “Recopy-inhibited” is not permitted. However, since the accumulation medium is limited in capacity, the content with the copy control information of “Recopy-inhibited” after being accumulated is allowed to move. In moving the content for the above case, it is necessary to render playback of the source content impracticable, after the moving is completed. Also in the course of moving operation, the content having a length of over one minute should not be capable of being played back simultaneously on either the moving source side or the moving target side (according to the operational rules of ARIB; Association of Radio Industries and Businesses). As thus described, in moving the content, it is significant to control so that the volume of replayable content is not increased.
For instance, there are some methods which render playing back of a source content impracticable in the moving process. The first method is to sequentially erase the content data itself, which has already been moved. This method is carried out, for example, by overwriting the moved content area with particular data. The second method is to split the content into multiple regions and store the content with encryption keys varied by region. Then, only the key corresponding to the region having been moved is erased. With the second method, since the data to be erased is the key only, it is possible to sufficiently reduce the processing load, compared to the first method.
The third method is disclosed in JP-A No. 2003-101529. As shown in FIG. 2, this method is to split the content into multiple regions, and one region is provided to store keys (Keys 0 to 5) to encrypt the content. Then, by use of the key stored in the region for storing the keys, the contents is decrypted, and a new decrypting key is created based on the data obtained by decrypting the content. Subsequently, the newly created key is used to decrypt the content stored in the next region.
With the third method, once the region for storing the key stores a key to decrypt the first region of the content, a key for decrypting the next region can be created from the decrypted content data. Therefore, unlike the second method, it is not necessary to store keys with respect to each region of content. Furthermore, if playback of the content is rendered impracticable, the newly created key is stored in the region for storing the key. Accordingly, the content in the regions earlier than the current one cannot be decrypted, and the playback thereof is rendered impracticable.
According to the related art as described above, when the content of “copy-one-generation” is recorded on the accumulation device which is built in a device such as HDD (Hard Disk Drive), the content is set to be “Recopy inhibit”. In the case where the content of “Recopy inhibit” is moved to a removable medium such as DVD, if the moving operation is interrupted in the course thereof, due to a power failure or a defect in the removable medium, playback of the contents on both the source side and the target side is rendered impracticable simultaneously, and thus there is no problem from the viewpoint of copyright protection. However, it results in the content itself being split into two parts of the source content and target content.
When a user tries to view the content in the status above, the user has to select the source content again after viewing the target content, if the user wishes to view what is continuing from the target content, and it is inconvenient for the user. After a lapse of long time, there is a possibility that the user may forget the correspondence between the source content and the target content, and the user may be forced to view a large number of contents little by little to search out the continuing content. Therefore, it is time-consuming and inconvenient.